add CRBB support to write_packet_ack_nack_desc_egprs

According to TS 44.060 §12.3.1 EGPRS Ack/Nack Description can use compressed block bitmaps.At the moment there is todo code plug which forces use of URBB makes coverity unhappy (CID 1351734).Note: this would require extending osmo_t4_encode() into proper encoder with backtracking instead of current "best guess" approach.